French Pat. No. 2,166,448 granted to the applicant describes a system of feeding oil to a 2-cycle engine, which system also incorporates a means for interrupting the flow of the oil at rest, essentially composed of a centrifugal valve located downstream from the pump, acting against a means of elastic return and closed when at rest under the action of the latter.
According to a specific embodiment described in the above-cited patent, the centrifugal valve is mounted on a part which rotates around a shaft and an oil feed channel is provided in this part. The successive sections of this channel are located on the radii of gyration which, in the direction of flow of the oil, are constant or tend to increase, but never decrease.
When the part turns, centrifugal force operates on the one hand to open the valve and on the other hand causes the drops of oil to travel along the feed channel, these drops being subjected to a centrifugal force field which never decreases as they travel along their route.
Taking for example a small-capacity motorcycle engine, for example 50 cc, the oil flow rate will be on the order of 7 to 14 cubic mm per second. With a feed channel with a section of about 3 mm.sup.2, this flow would amount to an average oil velocity of about 2-5 mm per second, disregarding the influence of centrifugal force.
In reality, however, this centrifugal force reaches higher values. Thus, for a rotational velocity of the rotating part on the order of 3,000 rpm, the acceleration produced by this force is about 250 g (g being the acceleration due to gravity) at a distance of 2.5 centimeters from the center.
Under these conditions, the oil, which is fed in small amounts by the viscosity-type feed pump, is very strongly accelerated by the centrifugal force and there is not a continuous column of oil in the feed channels. As a result, the pressure of the gases contained in the engine crankcase where the oil is distributed is exerted right up to the outlet of the feed pump.
This system operates satisfactorily in an engine in good condition. In fact, the crankcase gases are subject to alternations in pressure of about 0.5 kilograms per square centimeter for example during the operating cycle, and to rarefaction which can reach 0.35 to 0.40 kilograms per square centimeter. On the average, the crankcase gases are therefore at a slight overpressure with respect to the atmosphere.
However, engines, and especially motorcycle engines, are often poorly maintained, and the exhaust manifold as well as the muffler may have deposits such as calamine. The load loss in the exhaust therefore increases and causes an increased gas pressure in the engine crankcase. This higher pressure is insufficient to counterbalance the centrifugal force in the feed channel, but when it spreads to the feed pump, whose outlet pressure hardly exceeds 0.2 to 0.3 kilogram per square centimeter, it can interrupt the flow of the latter and dry up the lubricating device.